At bottom, a strobe and high-speed camera accurately represent the way in which a string is moving. At top, a video taken with an iPhone camera distorts your sense of how the string is moving by capturing instead images of standing waves, caused by the rolling shutter on the device. The video isn’t wrong – it’s just showing you beautiful visualizations of standing waves that make visible how the shutter works on the camera more than they do how the guitar works.

Full disclosure: I love waves. Analog, digital, acoustic, we’re talking vibrations in sound (and other substances, as well — and light). I don’t think you get into this area without having a certain wave addiction. If you love waves, you could easily get lost in exploring videos of vibrating guitar strings and pondering the physics of the string.

This story begins not with how guitar strings actually vibrate, but a curious phenomenon when combining the regular oscillation of the string with the rolling shutter of a CMOS digital camera — namely, the iPhone’s. To accurately capture motion, you need to record an image all at once (or at least come close). Rolling shutter or line scan as exhibited in a CMOS camera sensor like the iPhone is a side effect of the capture being scanned from top to bottom, so the bottom portion of the image is saved later than the top. That causes motion to skew across the image. (Long before digital, people played around with the same effect in analog video and even using film photography – all you need is something moving and a way of capturing the image that moves gradually in a different direction.)

When the regular oscillation of the scanning combines with the oscillation of what you’re filming – as with a vibrating guitar string, or the rotating propeller on an airplane — the two frequencies effectively phase, causing some curious distortion. In the case of the guitar, this means seeing the appearance of standing waves that, while they can occur in nature, don’t occur on any conventional guitar. (You can also think of the basic effect as aliasing, as seen optically when video shutters capture the frequency of rotation of a rotating car wheel in such a way that it appears to move backwards.)

As with many concepts in physics, it’s all easier to see than explain, so I’ll turn it over to some terrific videos. I’ve contrasted two at the top of the story; here are more examples.

Below, a high-speed camera operating at 600 and 1200 frames per second, played back 20x and 40x, respectively, slower than you’d see with your naked eye.

And here’s another example of how that might appear on a camera like the iPhone:

Of course, that means – fodder for our sister site Create Digital Motion – potential for more creative abuse, beyond the mere novelty.

What’s also amusing is the heated discussion this triggered as the iPhone video went viral. Read some angry comments – and some solid science among them — at Reddit:
Guitar string oscillations captured on video []

I’m glad people don’t behave (yet) in person the way they do on the Internet.

In person: “Excuse me; I think your fly is open.”

On the Internet: “That guy’s fly is open. He doesn’t even know his fly is open. That’s bullshit. I mean, what kind of person leaves their pants just *(&$#ing open like that? Look, look, look at his open fly. I’m never talking to him again. I’m not even going to wear pants from now on.”

(Seriously, happily, many of the comments are perfectly polite and well-informed!)

School’s out for just about everyone, but I’m going to remember to file this away for the next time I have to explain sound vibration. Now, back to the beach, or wherever you’re relaxing.

Thanks to Alexander Chen, from whom I lifted this via Google+. (See his own work on CDM here and here. Alex is just the kind of person I want to see this, as he’s been working with the aesthetics of vibrating strings! So, hurrah, Google+…